In the automotive field, for example, improvement in collision safety is required together with improvement in fuel consumption by weight reduction of vehicle bodies for environmental conservation. Thus, use of high-strength steel sheets for thickness reduction and optimization of vehicle body structures have been practiced hitherto in various ways, so as to achieve weight reduction of vehicle bodies and improvement in collision safety.
Large fatigue strength is also required in the high-strength steel sheets for weight reduction of vehicle bodies. In general, the fatigue strength of a parent material to be used for a welded member increases in proportion to steel sheet strength, but it is known that the fatigue strength of a welded joint barely increases even when the steel sheet strength is increased. This hinders the weight reduction of vehicle bodies by use of the high-strength steel sheets.
For underbody members such as suspension arms and sub-frames in particular, the fatigue strength of welded portions becomes more important. Fillet arc welding is often used for welding of these underbody members. Thus, in order to achieve the weight reduction of underbody members, an increase in fatigue strength of fillet arc welded joints becomes an issue.
FIG. 1 shows a cross-sectional shape of a typical lap fillet welded joint formed by overlapping an upper steel product 1 and a lower steel product 2 and welding only a corner portion of one side of corner portions formed on both sides of an abutted portion of the upper steel product 1 and the lower steel product 2. In such a lap fillet welded joint, stress is concentrated in a toe portion 4 and a root portion 5 of a fillet weld bead 3 and a fatigue crack progresses in a direction vertical to a load, and thereby the welded joint is broken. Therefore, for improvement in fatigue strength, decreasing the stress concentration in the toe portion 4 and the root portion 5 becomes important. Incidentally, in the following explanation, the fillet weld bead is referred to as a fillet bead as necessary.
Conventionally, as a means of improving fatigue strength of a fillet welded joint, welding a reinforcing member such as a rib into/to a proper shape ⋅ position, finishing a toe portion of a weld bead by a grinder operation, decorative build-up welding, and the like, and the like are performed. However, welding an additional member leads to an increase in cost. Further, additional work is required for the finishing of the toe portion. Thus, these means are not techniques applicable to manufacture of mass-produced goods such as automobile parts.
Further, in terms of a welding method, a technique of decreasing stress concentration to a toe portion is proposed in Patent Literatures 1 and 2.
In Patent Literature 1, there is disclosed a method of decreasing stress concentration to a toe portion and improving fatigue strength by optimizing chemical components of a weld metal to increase a curvature radius of the toe portion.
In Patent Literature 2, there is disclosed a weld bead structure in which a weld bead is extended when an end surface of another steel sheet is butted against one surface of a steel sheet to have a T-shaped cross section and a fillet bead is formed on both sides of the butted portion.
In Patent Literature 3, there is disclosed that when a sheet surface of a plate-shaped member and a square member are butted and fillet welding is performed with respect to all the portions of edges of the square member in contact with the plate-shaped member, linear welded portions intersecting crosswise each other are formed on corner portions of the square member.
However, even with the techniques disclosed in Patent Literatures 1 to 3, it is not possible to expect the effect of decreasing the stress concentration to the root portion 5 of the typified lap fillet welded joint formed by welding only one side of the overlapped portion of the steel sheets 1 and 2 shown in FIG. 1.
Further, in the technique disclosed in Patent Literature 2, the weld bead is extended, and thereby fatigue strengths at a welding start point (start edge portion) of the weld bead and at a welding end point (end edge portion) of the weld bead improve. However, there is little effect for improvement in fatigue strength of an intermediate portion of the weld bead.
Further, in the technique disclosed in Patent Literature 3, the number of welding start points to remain independently near a fillet bead is increased. The shape of a toe portion at a welding start point projects and this projected angle is steep, so that stress is easily concentrated at the welding start point if the welding start point exists independently.
As above, in the techniques disclosed in Patent Literatures 1 to 3, it is not easy to suppress occurrence of fatigue fracture when a welded structure member to which a cyclic load is applied such as an automobile underbody member is formed by fillet welding metal sheets having a thin sheet thickness.